Non-volatile memory cards (NVMCs) are becoming more and more popular. They are now widely used in many applications, notably in storage solution for digital consumer devices such as digital cameras or personal audio equipment. In such applications the data stored on the memory card is usually copied to another device like a personal computer or from another medium like an Audio-CD, which serves as a long-term storage facility. Consequently long-term storage of data on non-volatile memory cards has not been an issue yet.
As prices for such memory cards go down, they are also used in other application areas, e.g., for long-term storage of all kinds of data. Although non-volatile memory cards provide a reliable storage solution, even such memory cards degrade with time. For that reason it is necessary to keep track of the age of non-volatile memory cards.
If a memory card reaches a critical age, the memory module's data content needs to be refreshed, e.g., by reading it into the volatile memory of a card controller or a host system and writing it back to the memory module. This procedure activates memory card internal memory management functions, for example bad block detection and exclusion. Alternatively, it is possible to copy the data from the old memory card to a new memory card and to dispose of the old memory card.
With current technology, it is possible to estimate the expected lifetime during the manufacturing process or use of the memory card, e.g., based on statistical data from quality tests like counting the number of defective storage cells. However, it is a disadvantage of the current generation of non-volatile memory cards that it is very hard to enforce the deactivation of a non-volatile memory card at the end of the expected lifetime once it has been released to a customer. Usually constraints concerning the lifetime of a memory card are just printed as a disclaimer or guaranty on the packaging of the card. However, many customers do not read such information or do not act according to them. In addition such disclaimers are typically based on indirect aging parameters like storage cycles or the mean time between failures (MTBF), which bear no direct meaning for a consumer.
One method of overcoming the problem is to specify the number of allowed read and write cycles of the non-volatile memory card. Because every read and write operation is performed by a card internal controller, a counter can be implemented in the controller and can be used to record the number of read and write operations performed on the non-volatile memory module.
Alternatively, the operational lifetime of the non-volatile memory card can be determined by measuring the time the device has been active, i.e., the time the memory card was connected to a host system providing the memory card internal controller with electrical energy.
Unfortunately these methods are not as flexible as measuring the wall clock time directly. An additional disadvantage of these methods is that such observations can only be made and analyzed as long as the non-volatile memory card is connected to a host system. Consequently, such methods are unsuitable for long-term storage solutions based on non-volatile memory cards, such as data archives comprising a multitude of NVMCs, which are only connected to an external, electrical device very infrequently. For such applications the number of read and write cycles or the operational lifetime is very low and not suitable to evaluate the device's lifetime.